This invention relates to improvements in or relating to the manufacture of face plates for television tubes and particularly to face plates which are resistant to browning. One aspect of the invention relates to the production of such face plates by a process in which molten glass is formed into a sheet from which the face plates may be made by flowing the molten glass onto a bath of molten tin.
The use of a molten tin bath for the forming of glass suitable for use as the glass from which T.V. or X-ray absorbing face plates can be formed has been described in e.g. U.S. Pat. No. 4,015,966. X-ray absorbing glasses have been of importance since the advent of television and such glasses must contain materials capable of absorbing X-rays, and of preventing browning due to the absorption of such X-rays in the glass. The inclusion of cerium oxide in the earliest glasses used for the face plates of television sets was described in UK Pat. No. 664769. Such glasses however also contained either or both As.sub.2 O.sub.3 and Sb.sub.2 O.sub.3 as refining agents. As indicated in U.S. Pat. No. 4,015,966 such refining agents cannot be present in a molten glass which is to be formed on a molten tin bath. This is because in order to avoid the formation of tin oxide on the surface of the molten tin and in the area surrounding the molten tin, the forming process is carried out in a chamber where the molten tin is protected by the presence of an atmosphere containing a percentage of hydrogen. Thus if sufficient reducible metal oxides are present in the molten glass fed to the surface of the molten tin they will tend to be reduced to a metallic state giving a surface stain in the finished glass. It is thus not possible to use As.sub.2 O.sub.3 and/or Sb.sub.2 O.sub.3 as conventional refining agents in a glass to be formed on molten tin in the presence of a reducing atmosphere.
U.S. Pat. No. 4,015,966 suggests that "Refining agents such as fluorides, nitrates and sulphates can be used in place of the arsenic and antimony".
We have now found that it is not possible to achieve a satisfactory degree of refining using nitrates or sulphate alone, and we prefer to avoid the use of fluorides and nitrates in a glass to be formed on molten tin as they can have an adverse effect on the process.